Laser (light amplification by stimulated emission of radiation) has revolutionized optical science and technology, but the microwave counterpart thereof, maser (microwave amplification by stimulated emission of radiation) has not reached the same level of use. A main difficulty is that all existing masers demand harsh working conditions that limit the maser applications. For example, gas masers require a high vacuum, free electron masers require large accelerators, and solid-state masers typically function only at cryogenic temperatures. The only existing room-temperature solid-state maser is based on pentacene-doped p-terphenyl molecular crystals (Oxborrow et al., Nature 488, 2012). This pentacene maser has a high pump threshold (˜230 W) and operates only in the pulse mode. In addition, the organic crystal material of the maser is unstable under a strong optical pump.